Pulsed Laser System with Saturable Absorber

ABSTRACT

A high power pulsed laser system is configured with at least two gain blocks and with at least one saturable absorber (SA) coupled to the output and input of the respective gain blocks. The SA is configured so that Qsat_sa&lt;Qsat_gb, wherein Qsat_sa is a saturation energy of the SA, and Qsat_gb is a saturation energy of the gain blocks. The SA is further configured with a recovery time τ&lt;1/f providing for the substantially closed state of the SA, wherein the f is a pulse repetition rate, and with the recovery time τ smaller than a round trip time Tround_trip=2*(L 1 +L 2 )*n/c, where L 1 , L 2 —lengths of the respective gains gain blocks, n—a refractive index of active media, c—a speed of light in vacuum.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates to a pulsed, high power optical system. Inparticular, the disclosure relates to a pulsed multi-cascade fiberoptical system configured with at least one passive modulator which isoperative to isolate the consecutive gain blocks of the optical system.

2. Prior Art

One of techniques, allowing increasing the energy level of the outputoptical pulse generated by a pulsed-operated laser system, includescascading multiple gain blocks or simply amplifiers. The gain factor ofthe cascaded laser system corresponds to the multiplication of the gaincoefficients of all blocks which may be, thus, high. However, the highgain of the output may be unacceptable for the following reasons.

One of the reasons includes the presence of amplified spontaneousemission (ASE) which is proportionally amplified by the gain factor ofthe system and, thus, may reach unacceptably high levels. Theconsequences of high levels of noise—the ASE is no more than a uselesssignal or noise—include, for example, the pronounced inefficiency of alaser system. Still a further consequence is associated with the verypresence of the high energy levels of the ASE at the output of thesystem which may be unacceptable in a variety of laser system'sapplications. For example, the presence of the high-energy ASE cannot betolerated in marking sensitive materials, such as plastic.

A further reason explaining detrimental effects of high gain factor tothe operation of the pulsed laser system includes a so-called selfpulsing phenomenon causing the degradation and possible destruction ofamplifiers or gain blocks, particularly those configured from fiber.Still another reason for avoiding high gain relates to a buildup ofbackreflection unavoidable in practical applications of the system.Concomitantly, backward Rayleigth scattering, Brillouin scattering aswell as other scattering noise developed in the cascades and propagatingthrough the laser system also contribute to the degradation of thesystem.

As a result, as readily realized by one of ordinary skills in thepulsed-operated laser art, the gain blocks should and are typicallyoptically isolated. The means used for isolating cascades are brieflydiscussed immediately below.

In accordance with one approach, adjacent gain blocks may be mutuallyisolated by a bulk isolator. The bulk isolator is configured to transmitradiation in one direction and block it in the opposite direction. As aconsequence, while the problem with backreflected noise may beadequately addressed, the forward propagation of noise still poses aproblem.

The other approach includes the use of active modulators configured tomanipulate properties of light beams, such as the optical power orphase. The active modulators include, among others, acousto- andelectro-optic modulators. The former, as well known to the artisan, isbased on the acousto-optic effect, while the latter exploits theelectro-optic effect. The active modulators thus operate as asynchronously timed gate employed between adjacent gain blocks. Theactive modulators have certain advantageous over bulk isolators. Forinstance, the forward absorption of the ASE to a subsequent, downstreamgain block is prevented, except for a temporary open window ofmodulator. As a consequence, the resulting noise at the output of thelaser system is substantially suppressed. However, the use of activemodulators adds significant cost and complexity to the laser system andleads to a less robust and bulkier structure.

One of the solutions of the above-discussed problems includes the use ofsaturable absorber which is briefly mentioned by W. V Sorin and et al.in a paper titled “Single-mode-fiber saturable absorber” incorporatedherein by reference in its entirety. However, to the best knowledge ofApplicants, neither the incorporated paper, nor any other known toApplicants source suggests the optimization of the saturable absorber ina multi-cascaded high power laser system.

A need, therefore, exists for providing a cost-effective multi-cascadedpulsed-operated fiber laser system which is provided with an optimallyconfigured saturable absorber capable to minimize the above-discussedproblems.

SUMMARY OF THE INVENTION

The foregoing need is met by incorporating a passive modulator in thedisclosed pulsed-operated multi-cascaded laser system. In particular,the passive modulator is configured as a saturated absorber (SA).

In accordance with one aspect the disclosed SA is configured with aspecifically selected recovery time period “τ”. The closed ornear-closed state of the SA between pulses, during which the propagationof light is suppressed, is achieved by configuring the SA with therecovery time period τ smaller than 1/f, wherein f is a maximal pulsefrequency. To further ensure the closed state of the SA, the recoverytime period of the SA is smaller than a roundtrip time T correspondingto forward and backward propagation of radiation between adjacent gainblocks.

In accordance with a further aspect of the disclosure, a SA isconfigured as an output isolator. In this configuration, theconfiguration of the SA is optimized so that it substantially minimizesthe propagation of backreflection which is unavoidable in the industrialenvironment.

Still a further aspect of the disclosure is concerned with a buildup ofscattering noise, such as ASE, propagating through a pulsed systembetween consecutive pulses is substantially. The configuration of theoutput SA is optimized to block ASE from reaching a downstream gainblock.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of one embodiment of the disclosureillustrating a saturable absorber which functions as an isolator.

FIG. 2 is a diagrammatic view of a further embodiment with a saturableabsorber configured to operate as a backreflection isolator.

FIG. 3 is a graph illustrating the dependence of the absorber'stransmissivity from an input power.

FIG. 4 is a graph illustrating the dependence of the absorber'stransmissivity from an input energy.

FIG. 5 illustrates a square pulse coupled in to the input of theabsorber.

FIG. 6 illustrates the transmissivity change in the saturable absorberin response to the square pulse of FIG. 5

FIG. 7 illustrates the output pulse at the output of the absorber.

FIG. 8 illustrates the relaxation process in the absorber betweenadjacent pulses.

FIG. 9 illustrates a sequence of pulses coupled into the input of thedisclosed saturable absorber.

FIG. 10 illustrates the transmission process occurring in the disclosedsaturable absorber in response to the sequence of pulses of FIG. 9

SPECIFIC DESCRIPTION

Reference will now be made in detail to the disclosed system. Whereverpossible, same or similar reference numerals are used in the drawingsand the description to refer to the same or like parts or steps. Thedrawings are in simplified form and are far from precise scale.

FIG. 1 illustrates an optical pulsed system 25 including two or moreoptical amplifiers or gain locks 20, 22. To increase the output power ofgain blocks 20, 22, respectively, which is a current trend in the laserindustry, the gain blocks each should be capable of storing as muchenergy as possible. Hence, the greater the stored energy, the greaterthe output power of the block. To provide for the effective operabilityof system 25, system 25 further includes at least one saturable absorber24 coupled between output and input of respective blocks 20 and 22. Inaccordance with the disclosure, SA 24 has an optimal configuration forthe effective optical isolation of gain blocks 20 and 22.

The principle of operation of SA 24 is based on the saturation of lossby ions implanted in accordance with known techniques. With lightimpinging on SA 24, it is absorbed by at least some ions located on theground level which thereafter make transition to a higher level(s).However, the greater portion of light passes (is transmitted) through SA24. With the intensity of light increasing, the population of exitedions is also increasing, the phenomenon known as bleaching or a SA openstate. Accordingly, the open state is characterized by minimalabsorption and power losses which the light pulse experiences whilepropagating through SA 24. The lower absorption, the lower losses, thehigher the light transmission through SA 24.

As illustrated in FIGS. 3 and 4, the absorption coefficient orpercentage of light absorbed depends, among others, on the type ofdopants and their characteristics and, of course, the inputpower/energy. As a pulse is coupled into the input of SA 24, initiallyits leading front experiences high losses. As the portion of absorbedenergy in SA 24 increases, the transmission of light abruptly increasesto its maximum while, of course, the losses decrease reaching itsminimum. These conditions correspond to an open state of SA 24 and gainblocks 20 and 22 are optically coupled.

With the light intensity decreased, more and more previously exited ionsreturn to the ground level. As a consequence, the transmission isgradually decreases and SA 24 assumes a closed state waiting for thenext pulse. In this state, the light transmissivity T₀ through SA 24should be substantially nonexistent. However, to operate in the desiredopen/closed sequence, SA 24 should be correctly configured.

Turning back to FIG. 1, typically, gain blocks 20 and 22 each are pumpedto acquire a high gain factor G and, thus, to amplify a coupled pulse.If system 25 were configured without SA 24, the overall gain of system25 would be the sum of gain factors G₂₀ and G₂₂ of respective gainblocks 20 and 22. However, in reality, any laser system is alwaysassociated with parasitic backreflection since light may bounce backupstream from any internal “impurity” such as miniscule defects within again block itself, surfaces of bulk elements, Rayleigh, Brilloun andother types of scattering, splices between fibers and etc. Thus, acombination of amplifying cascades or gain blocks 20, 22, respectively,always have a feedback forming a resonant cavity.

The presence of the resonant cavity poses a serious problem when itstotal losses exceed the gain (+dB) which the light experiences duringits forward and reverse propagation or a round trip Tround_trip.Configuring a powerful laser system with minimal reflections whichgenerate −dB compensation for the total gain is practically impossible.Accordingly, when laser system 25 is configured without SA 24, the totalgain G (+dB) exceeds losses (−dB). As a consequence, system 25 becomesnot a two gain block system, but a single laser generating unit.Accordingly, SA 24 should be configured so that it adds additionallosses which, when added to the losses of the system without theabsorber, will allows the losses exceed the gain. In this case, thegeneration does not occur.

The SA 24, thus, is configured so that it introduces sufficient lossesall the time except for short period of time Topen when it is in theopen state and its losses, thus, are practically nonexistent. Undercertain conditions imposed upon Topen, as discussed hereinbelow, SA 24is operative to prevent gain blocks 20 and 22 from generating lightemission in the open state thereof. When SA 24 is switched to the openstate, gain blocks 20 and 22 are already operating as a laser unit andstart to generate emission. But to obtain the latter, it is necessary tohave some time during which the emission is built. This time correspondsto a round trip (or trips) along a resonant cavity including gain blocks20, 22 and can be determined as follows:

Tround_trip=2*(L1+L2)*n/c  (0)

where L1, L2—lengths of the upstream and downstream gain blocks 20 and22, respectively, n—refractive index of an active media, c—speed oflight in vacuum. Thus, SA 24 is configured so that Topen<Tround_trip. Inthis case, system 25 is incapable of becoming a laser and the presenceof SA 24 is justified.

FIGS. 5-8 illustrate the operation of SA 24 in system 25 operating withsquare-shaped pulses 26 at the output of, for example, gain block 20 ofFIG. 1. As disclosed above, the leading front of pulse 26 issubstantially suppressed for a short period of time necessary to reducethe absorption and, thus, increase the transmission of light as shown inFIG. 6. Accordingly, the losses, which the leading front of pulse 26experiences, are illustrated by a wing 27 of a pulse 26′ at the outputof SA 24 of FIG. 7. Once the transmission T reaches its maximum (1)(FIG. 6), the greater part of pulse 26 is losslessly transmitted asillustrated by output pulse 26′ of FIG. 7.

Referring to FIG. 8, as the pulse has passed through SA 24, the lattergradually assumes a closed state because exited ions require some timeto return back to the ground level. The time, known as recovery time τis, generally speaking, the time required for SA 24 to assume a fully ornear closed state. The recovery time is thus an important parameter ofSA 24, and as will be shown below, imposes certain restriction on themaximum frequency f of the pulse generation (pulse repetition rate PRR).

Referring to FIGS. 9 and 10, for SA to assume closed or near closedstate, it should be configured so that τ<1/f (FIG. 9), where τ—therecovery time and f—pulse frequency (PRR). Otherwise, SA 24 cannotassume a closed or near closed state after leading pulse 26 has passed,and the absorption of SA 24, at the time trailing pulse 26 is coupledinto the absorber's input, is lower than the absorption of SA 24 in itsclosed state at T₀ in FIG. 10. Thus, for recovery time τ of 1microsecond [μs], a maximum pulse repetition rate (PRR) should notexceed about 1 MHz.

Furthermore, in accordance with this disclosure, SA 24 is configured sothat its recovery time τ is smaller than the round trip timeTround_trip. The latter, of course, imposes limitations on the geometryof system 25, particularly if system 25 including SA 24 is configured asan all fiber system.

Typically, the length of fiber in system 25 configured in accordancewith FIG. 1 varies between about 1 m and 10 m which corresponds to roundtrip time Tround_trip from about 10 nanoseconds to 100 nanoseconds. Witha range of above-mentioned conventional lengths, the recovery time τshould also lie in a nanosecond diapason. To relax such a stringent timelimitation, system 25 may have an additional fiber loop 28 (FIG. 1)configured from a passive (undoped) fiber. The addition of loop 26around 100 m will increase the round trip time to about 1 microsecond.Hence, the length of fiber SA 24 should be taken into consideration toprovide the effective isolation of gain blocks 20, 22, respectively.

The suitable configurations of the SA, besides fiber, may include, amongothers, semiconductor and other crystals. Other saturable absorbers maybe configured from a length of fiber doped with ions. Fiber saturableabsorbers are particularly advantageous because, in contrast tocrystals, it is easy to select the desired absorption changing thelength of the fiber.

One of the limitations of SA 24 affecting the effective isolationbetween gain blocks 20 and 22, respectively, relates to the energy ofsaturation Qsat. The saturation energy of the gain/loss medium is theenergy of incident optical pulse which leads to a reduction in thegain/loss to 1/e (i.e. 37%) of its initial value. In accordance with thedisclosure, the effective isolation occurs when SA 24 meets thefollowing condition

Qsat_sa<Qsat_pa

where Qsat_sa—the saturation energy of SA 24, Qsat_pa—the saturationenergy of each gain block. If this condition is not met, then the lossesof power in SA would be unjustifiably high. In other words, in order tocompensate power losses of pulse 26 in SA 24, the gain factor of thegain block should be increased, but the absorption of SA in closed state(unsaturated absorption) could not compensate that gain increasing. Inthis case, the integration of SA 24 into system 25 would not make anysense. In short, the lower the saturation energy of SA 24, the smallerthe losses in the SA. The saturation Qsat_sa of SA 24 can be affected bythe type of dopants which may be selected from the group comprising ionsof transition metals Cr and V, ions of Bi, ions of Sm and other suitableelements. In particular, the dopants are selected based on σ_(es)

σ_(as)—cross-sections of emission and absorption, respectively, at thedesired wavelength. Furthermore, saturation energy Qsat_sa is directlyproportional to the active area of SA dopants multiplied by mode overlapfactor.

Assume that the pulse energy at the output of gain block 20 and, thus,at the input of SA 24 is determined to be q, whereas the pulse energy atthe output of SA 24 is q₁. If the relaxation time τ of SA 24 issubstantially longer that the duration of the generated pulse, i.e., τ>Tpulse, then, as the pulse passes through SA 24, the relaxation processmay be neglected and, therefore, energies q

q₁ of pulse at the absorber input and output, respectively, relate toone another based on the known Frantz Nodvick equation:

$\begin{matrix}{{q\; 1} = {{Qsat\_ sa} \cdot {\ln ( {{T_{0} \cdot ( {^{\frac{q}{{Qsat}\_ {sa}}} - 1} )} + 1} )}}} & (1) \\{{Qsat} = \frac{Shv}{\Gamma ( {\sigma_{es} + \sigma_{gs}} )}} & (1.1)\end{matrix}$

where S—the square of active zone, σ_(es)

σ_(as)—cross-sections of emission and absorption, respectively,T₀-unsaturated transmission, i.e. the transmission corresponded toclosed state. Γ—overlapping factor between mode and active zone of SAFrom equation 1, the absorbed energy of the pulse can be determined asfollows:

$\begin{matrix}{{\Delta \; q} = {{q - {q\; 1}} \leq {{Qsat\_ sa} \cdot {\ln ( \frac{1}{T_{0}} )}}}} & (2)\end{matrix}$

When the saturation energy of absorber Qsat_sa is substantially smallerthan the energy of pulse q at the input of SA 24, the energies of thepulse at the input and output of SA are substantially equal to oneanother. Based on equation (2), it is evident that the smaller Qsat_sa,the smaller the energy absorbed in SA 24, the faster SA 24 opens.

When the limitation τ>>Tpulse cannot be met, it is necessary toconsiderer the relaxation process in dynamic equations describing thesaturation of the pulse. In this case, the saturation energy can bedetermined in accordance with the following equation:

$\begin{matrix}{{\Delta \; q} = {{q - {q\; 1}} \leq {{Qsat\_ sa} \cdot {\ln ( \frac{1}{T_{0}} )} \cdot ( {1 + \frac{Tpulse}{\tau}} )}}} & (3)\end{matrix}$

The saturation energy Qsat_sa in fiber absorbers may be altered bymodifying a central doping profile to a ring-shaped profile for a givenmode filed diameter (MFD) The saturation energy Qsat_sa can also bealtered by varying of MFD for given doping profile. The transmissioncoefficient T₀ (FIGS. 3, 4) in the closed state of SA 24 is independentfrom its saturation energy Qsat_sa and, in general, is selected based onspecific requirements for the optical isolation between gain blocks ofany given system configuration. For fiber SA, the logarithm of thetransmission coefficient T₀ linearly depends from the length of thefiber and, thus, can be easily carried.

Up till now, the configuration of SA 24 has been considered withouttaking into consideration the possibility of the presence of smalloptical signals at the input of SA 24 in the closed state thereof.However, such a possibility is high and may affect the transmissioncoefficient T₀. One of the possibilities of amplified small signals maybe amplified spontaneous emission from gain blocks 20, 22 (FIG. 1)(ASE).

The transmission coefficient T of the SA 24 in the closed state, as weexpect, is close to the unsaturated coefficient T₀, However, it shouldbe noted that the absorption of SA 24 in the closed state depends on theASE at the input of SA 24 between consecutive pulses. To have thetransmission T close to T₀, which would correspond to the absence ofinput power, the following condition has to be met:

P_ase<P_sat_sa

where P_ase—mean power of ASE, a P_sat_sa=Qsat_sa/τ—is saturation powerof the SA

The overall gain factor G of system 25 during the closed state thereofis a function of G₁*G₂*T₀, where G₁ and G₂ are respective small signalgain factors of blocks 20, 22, and T₀ is an unsaturated transmissioncoefficient of SA 24. Accordingly, configuring SA 24 with a specificallyselected transmission/absorption coefficient, the gain factor G ofsystem 25 may be as small as desired.

As an example of the above, consider the amplification of nanosecondoptical pulses (T_(pulse)=20 ns) with the initial energy of 0.1 μJapplied to an Yb gain block which outputs the pulse with the energy of10 mJ at 1064 nm wavelength. An average gain factor of such an amplifieris about 50 dB. Due to the practical considerations in general andparticularly because of backreflection that can easily reach a 50 dBlevel that provides for the self pulsing phenomenon, such a systemshould desirably have more than one amplification stage, for example, itmay be system 25 of FIG. 1. The gain blocks 20 and 22, respectively, areisolated from one another by SA 24 which, for instance, is configured asa crystal V³⁺:YAG. Assume that an unsaturated transmission coefficientis T₀=10⁻⁵.

The following table illustrates basic spectroscopic parameters for theSystem 25 operating at 1064 nm wavelength.

Qw σ_(es) σ_(gs) T V³⁺: YAG NA 3.0*10⁻¹⁸ cm² 20.0 ns Yb: Ph 1.3*10⁻²¹cm² 4.4*10⁻²³ cm²   1.4 ms

Further assume that gain block 20 and block 22 are configured withrespective core diameters d _(—) _(gb20)=9 um, d _(—) _(gb22)=70 um. Tosimplify the example, assume also that the overlap integral Γ=1.

The saturation energy Qsatsa of SA 24, upstream gain block 20 anddownstream block 22 can be determined based on equation 1.1 and have thefollowing respective values: Qsat_sa=0.58 uJ, Qsat_pa₂₀=88 uJ,Qsat_pa₂₂=5.3 mJ Using equation 3, the maximum energy absorbed in SA 24will be Δq=13.3 uJ. Upon amplifying the energy of pulse in upstream gainblock 20 up to q_pa₂₀=100 uJ, the part of energy that is lost in SA 24will be 13.3 uJ: 100 uJ=0.133 (13.3%).

The following table illustrates the results of the necessarycalculations:

Input PA20 SA input SA24 SA output PA22 Output Saturation energy   88 uJ0.58 uJ  5.3 mJ Pulse energy 0.1 uJ 100 uJ 86.7 uJ 10 mJ Average gain  30 dB N/A 20.6 dB Small signal gain 32.7 dB  −50 dB 25.3 dB

As can be seen from the above, system 25 may have a gain factor of 32.7dB-50 dB+25.3 dB=8 dB. The system may even have a negative gain factor,if the T₀ coefficient is sufficiently decreased.

FIG. 2 illustrates a further aspect of the disclosure, in accordancewith which a SA 30 is configured to function as an output isolatoroperative to minimize and even eliminate backreflection which inevitablyexists in practical applications of laser system 25. Similar to the SAdisclosed in reference to FIG. 1, SA 30 is configured with a recoverytime smaller than a round trip time necessary for a pulse to propagatefrom SA 30 towards an external surface and, upon being reflected, backto SA 30. The output SA 30 may be used simultaneously with SA 24, asshown in FIG. 1. Alternatively, output SA 30 may be used in lasersystems which are not configured with interstage saturable absorbers.

Assume that SA 30 is also configured as crystal V³⁺:YAG operative toisolate 10 mJ pulses in the forward direction. Assuming further that thebackreflection impinging upon SA 30 constitutes about 1% of the energycarried by the output pulse. Accordingly, if the energy of the outputpulse q=10 mJ, the energy of the backreflected signal q_(back)=10mJ*0.01=0.1 mJ.

If the crystal is selected with the coefficient of unsaturatedtransmission T₀=10⁻⁵ and the square S of the active zone is equal to thecore diameter of downstream block 22, then the energy of saturationQsat_sa=35.1 uJ. Using equation 1, the output pulse would loose only 4%while the losses in the backreflected pulse reach 42 dB.

Thus based on the disclosed technique for a saturable absorber, highpower laser systems will be characterized by the following advantages:

-   -   a. a relative small gain coefficient of the entire system which        provides for the improved reliability of gain blocks (in        particular protects system from self-generation);    -   b. substantially suppressed parasitic optical signals typically        present in the closed state of the absorber and proportional to        the total gain coefficient of the system which improves the        effectiveness of output gain blocks; and    -   c. a substantially decreased backreflection which allows for the        improved safety of upstream components of the laser system and        low power losses and stability of an optical pulse propagating        in a forward directions.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the presently disclosedlaser powerful system. For example, a laser system may have more thantwo gain blocks and, correspondently, have a saturable absorber betweentwo adjacent gain blocks. Thus, it is intended that the presentdisclosure cover the modifications and variations of this disclosureprovided they come within the scope of the appended claims and theirequivalents.

1. (canceled)
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 5. (canceled) 6.A pulsed laser system for processing an external surface, comprising: atleast one gain block operative to amplify a sequence of pulses; anoutput saturable absorber (OSA) coupled to an output of the gain block,the SA being configured with a recovery time τ smaller than a round triptime Tround_trip_br, which is necessary for a pulse to propagate betweenthe OSA and the external surface in opposite directions, wherein the OSAabsorber substantially suppresses a propagation of the pulse reflectedfrom the surface upstream toward the saturable absorber.
 7. The pulsedlaser system of claim 6 further comprising an other gain block upstreamfrom the one gain block and a second saturable absorber (SSA) coupledbetween the one and other gain blocks, wherein the SSA is configured asaturation energy that Qsat_sa smaller that a saturation energy Qsat-gb,wherein the Qsat_gb is a saturation energy of the gain blocks.
 8. Thepulsed laser system of claim 7, wherein the SSA is operative to have anopen state, in which the one and other gain blocks are optically coupledto one another, and a closed state, in which the gain blocks areoptically isolated, the SSA being configured with a relaxation timeτ<1/f so as to assume the substantially closed state, wherein the f is apulse repetition rate.
 9. The pulsed laser system of claim 7, whereinthe SSA is operative to have an open state, in which the upstream anddownstream gain blocks are optically coupled to one another, and aclosed state, in which the blocks are optically isolated, the SSA beingconfigured with a recovery time τ<Tround_trip so as to assume thesubstantially closed state, wherein the Tround_trip is a time necessaryfor a pulse to propagate between the other and one gain blocks inforward and backward directions.
 10. The pulsed laser system of claim 7,wherein the SSA is configured to have a power of saturation Psat_sagreater than a mean power of parasitic optical signals Pase which isgenerated from gain blocks and coupled into the SSA in the closed statethereof, wherein the Psat_sa is proportional to the saturation energy ofthe SSA and inversely proportion to the recovery time.
 11. The pulsedlaser system of claim 7, wherein the SSA is configured to have a closedstate between consecutive optical pulses in which the saturable absorberabsorbs an ASE generated in the other gain block so that the one andother saturable absorbers are isolated from one another in the dosedstate.
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